The present invention relates to new and improved methods for inhibiting inland atmospheric corrosion of steel, and particularly to improved sacrificial coatings for steel substrates.
In the past, ion vapor deposited aluminum has been proposed as a sacrificial coating to protect steel substrates from corrosion damage. Through salt fog exposure test such pure aluminum coatings have been shown to provide good potential for sacrificial corrosion protection of offshore steel substrates. However, evidence has been adduced to indicate that such pure aluminum coatings do not sacrificially protect inland steel substrates because the aluminum tends to passivate.
Our co-pending application discloses a method for solving this problem by alloying aluminum with other metals, anodic to steel, such as zinc and indium in a co-deposition process. Although the codeposition of aluminum-zinc alloys has met with extreme success, co-deposition of aluminum-indium alloys leaves a great deal to be desired for commercial success.
The periodical entitled "Corrosion" at Volume 40 , Number 7, July 1984, in the article "A Proposed Activation Mechanism For Aluminum Anodes" by the author's Reboul, Gimenez, and Rameau mentions that there is an exchange reaction between aqueous solutions of indium salts and aluminum to form metallic indium deposits on the surface of the aluminum. However, Reboul, et. al, believed that the indium had to be in solid solution in order to sufficiently activate the aluminum.
Many indium salts are unstable in aqueous solutions because of their tendancy to form insoluble compounds each as the hydroxide. Nevertheless, Indium is a highly desireable source for activating aluminum if a process could be developed to incorporate it into aluminum coatings more effectively from aqueous solution than by co-deposition, and achieve greater stability of an aqueous solution of certain of its salts.
Accordingly, a liquid solution activating process for aluminum coated steel substrates that would effectively permit sacrificial inland corrosion protection of steel in mild environments would be a surprising and unexpected advancement in the art, fulfilling a long felt need in the industry.